Report readdir failures

Report readdir failures

Previously, on non-MS-Windows platforms the code treated most
readdir failures as EOF.  This was incorrect, e.g., when readdir
fails with errno == EOVERFLOW.  Signal an error instead.
* dired.c (read_dirent):
New function, which signals an error when readdir fails.
(directory_files_internal, file_name_completion): Use it.